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Towards Efficient Monte Carlo Calculations in Reactor Physics

Criticality, Kinetics and Burnup Problems

Time: Fri 2021-11-26 10.00

Location: FD5, AlbaNova University Center, Roslagstullsbacken 21, Stockholm

Language: English

Subject area: Physics, Nuclear Engineering

Doctoral student: Ignas Mickus , Kärnenergiteknik

Opponent: Dr. Andrea Zoia, CEA Paris-Saclay

Supervisor: Jan Dufek, Kärnenergiteknik

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This thesis presents a compilation of work focused on Monte Carlo crit-icality, kinetics and burnup calculations in reactor physics. Performing suchcalculations usually comes at a high computing cost. Therefore, the main mo-tivation behind the presented work is lowering the computing cost of MonteCarlo calculations. To this end, three new methods for improving the comput-ing efficiency are proposed: a method for neutron population control in MonteCarlo criticality calculations; a hybrid stochastic-deterministic response ma-trix method for reactor kinetics calculations; and an optimisation method forMonte Carlo burnup calculations.

The first method gradually increases the neutron population size over thesuccessive cycles in Monte Carlo criticality calculations. This enables fasterfission source iterations at the beginning of a calculation where the sourcemay contain errors from the initial cycle while at the same time preventingthe source bias from dominating the error later in the calculation. The methodis tested on a set of full-core PWR criticality calculations.

The second method is based on the response matrix formalism which de-scribes a system by a set of response functions. The response functions arecomputed during Monte Carlo criticality calculations. These functions arethen used in a deterministic set of equations for solving a space-time depen-dent problem. The method is demonstrated on a set of absorber movementtransients in a PWR-type mini-core.

The third method sets the time step length and the number of neutronhistories simulated during each time step of Monte Carlo burnup calculationsaccording to the fraction of the computing cost assigned to the depletion solu-tions (and other procedures that are repeatedly executed before starting theactive cycles) and the overall computing cost of a Monte Carlo burnup calcu-lation. Optimal values of this fraction are studied in a set of test calculations.

Additionally, numerical tests on tally error convergence in Monte Carlocriticality calculations and stability of Monte Carlo burnup calculations arepresented. The context and the outcomes of the work are summarized inthe main body of the thesis while the details are presented in the appendedpublications.